The present invention is directed to an improved method of inhibiting corrosion of iron and iron-based alloys which are in contact with aqueous solutions. More specifically, the present method of inhibiting corrosion requires the use of adjacent paired or ortho dihydroxyaromatic compounds which contains at least one electron withdrawing group pendent from the aromatic ring. Besides being effective when used alone, it has been surprisingly found that a combination of the presently described dihydroxyaromatic compounds and certain conventional scale inhibiting agents dramatically enhance the effectiveness of corrosion inhibition.
Corrosion inhibition is necessary for protection of metal parts in equipment such as heat exchangers, pipes and engine jackets which are exposed to aqueous solution. Inhibitors are desired to prevent metal loss, pitting and tuberculation of such equipment.
Conventional corrosion inhibitors for iron and iron containing alloys each present certain drawbacks. For example, chromates are very effective but are very toxic, phosphates and organophosphonates can lead to scale deposition and are environmentally undesirable, zinc is not very effective at low levels (&lt;1 ppm) or at high pH (above 7.5) due to the limited solubility of Zn(OH).sub.2 and molybdates are generally not cost-effective. Thus, there exists a need for a non-chromate, non-phosphorous-containing, cost-effective corrosion inhibitor for iron-based metals.
Catechol and certain derivatives have been used in aqueous systems in attempts to inhibit iron corrosion. Japanese No. 58/133382 discloses the use of catechol as a corrosion inhibitor in association with calcium chloride brine, while Japanese No. 51/93741 uses it in ground water of 90 ppm total hardness and Japanese No. 48/71740 suggests using mixtures of catechol and phosphonic acids. Proc. Conf. Nat. Assoc. Corros. Eng., 26th Conf. 536-40 teaches that increased corrosion inhibition can be achieved by the introduction of an electron-releasing alkyl substituent on catechol. The corrosion inhibiting phenomenon observed was attributed to the surface acitivity and limited solubility afforded by a large hydrophobic group.
In certain applications, metal surfaces have been coated to resist corrosion. Japanese No. 61/78472 discloses coating iron material with epoxy resins containing catechol and its derivatives to provide a solid barrier against corrosion. However, coating of iron surfaces is not a viable approach to corrosion inhibition where the surface exposed to the corrosive aqueous media is internal to the system, and thereby not readily coatable; where the system would require enlargement of the apparatus to permit proper flow rate after coating; and/or where the coating would detract from the heat transfer efficiency. The above problems present themselves in many applications such as heat exchangers, boilers, cooling towers, pipes and engine jackets. Thus, there is a need for corrosion inhibitors which will work while dissolved in aqueous solution. These additives must be soluble, stable and active under operating conditions and these properties must not be adversely affected by the water composition or other conditions associated with such systems. These conditions include the presence of oxygen in the aqueous system which accelerates corrosion, the high degree of hardness associated with excessive amounts of calcium, magnesium and carbonate ions, as well as elevated temperature and pH conditions of these systems.